1. Technical Field
The present invention relates to a developing device and an image forming apparatus, and more specifically to a developing device and an image forming apparatus provided with the developing device.
2. Related Art
In general, image forming apparatuses such as a printer, a copier and a facsimile, which utilize electrophotography, are adapted to form a printing image on a recording medium such as a paper through a series of image forming processes including an electrifying step, an exposure step, a developing step, a transfer step, a fixing step and the like.
Such image forming apparatuses are provided with a developing device having a developing roller for carrying a toner thereon. The developing device is used in a state that the developing roller is arranged to face a photosensitive dram carrying an electrostatic latent image thereon. And, the developing device forms a toner image from the latent image by visualizing (developing) the same by applying the toner from the developing roller to the photosensitive dram.
Meanwhile, recently, from a viewpoint of formation of a printing image having high resolution, it is proposed to use small particle size toner particles each having a particle size equal to or smaller than 4 μm.
However, in the case of a dry type toner, if a particle size of each of toner particles contained therein would be merely set to a small value, the following problems occur. Namely, dispersal of the toner particles is likely to occur, and it becomes difficult to carry and convey the toner particles on the developing roller reliably.
On the other hand, conventionally, in order to prevent occurrence of a filming phenomenon of the toner on the photosensitive dram or the developing roller, there is known a toner which is constituted of resin base particles containing a binder resin and a coloring agent, and a small amount of oil added (externally added) to the resin base particles (for example, JP-A-2001-166527).
In such a toner containing the oil, even if a particle size of each of the resin base particles is set to a small value, since aggregates, which are formed by aggregation of the resin base particles via the oil, can behave as large particle size toner particles apparently, it is supposed that the above problems resulted from the use of the small particle size toner particles can be resolved.
On the other hand, however, in such a toner, since a particle size variation is likely to occur among the aggregates of the resin base particles, a charge property of the resin base particles becomes ununiform easily. As a result, there is a fear that a developing characteristic and a transfer property are adversely affected.
In particular, such an adverse affect is highly likely to occur in the small particle size toner particles. Therefore, even if the oil is merely added to the small particle size toner particles, it is difficult to form a printing image having high resolution.